The present invention relates to the connection of two elements of aluminum or aluminum alloys, where at least the first element is an outer layer forming part of a composite comprising a thermally decomposable core and at least one outer layer, and relates also to a process for manufacturing such joints. The invention relates further to the use of the process for manufacturing complex load-bearing structures that are mechanically stable and are of any desired form.
Composites, such as 3 to 200 mm thick sandwich panels made up of e.g. two outer layers of corrosion resistant aluminum alloys and a plastic core are finding ever increasing application in the building industry because of their favorable material properties such as low weight, corrosion resistance and almost unlimited shaping possibilities. In architectural applications for example, composite materials are employed as facade cladding, or composites with a thermally insulating core of plastic, mineral wool, foamed aluminum or foamed glass are employed as load-bearing facade elements, dividing walls or door fillers and roofing. Such light-weight, but rigid building elements featuring a core of aluminum honeycomb, balsa wood, foamed aluminum or plastic between two outer sheets of high strength aluminum alloys also find extensive application in aircraft manufacture and in the aerospace industry.
In the following text the term composite material always signifies a mechanically stable and load-bearing material having a core and at least one outer layer.
Metal constructions can be manufactured relatively easily by mechanical shaping or in the case of aluminum sections for example by extrusion, and assembled into complex structures by bolting, riveting, welding or adhesive bonding. Composites on the other hand are usually produced in the form of panels. Using composites for complex constructions involves therefore more extensive joining than metal constructions.
Assembling or joining various composite material construction elements into complex composite structures has taken place up to now essentially by bolting, riveting, adhesive bonding or, in the case of composite materials with non combustible cores or core materials that do not decompose at high process temperatures, also by welding.
Because of the space they require, mechanical attachment means such as bolts or rivets used to join together structural composite elements limit the possibilities for constructing complex structures. Consequently that also limits the use of composites for such applications.
Adhesive bonding on the other hand often requires special surface cleaning; the strength achieved is often quite limited or the area to be bonded has to be very large, which again limits the possibilities of construction.
Constructing with composites acquires greater flexibility if the structural elements are joined by welding. Because of the temperatures developed during welding, that method of joining can be considered only for composites with thermally stable core materials.
Up to now the welding of materials with less stable, i.e. thermally decomposable, core materials such as plastics has always failed as a result of the destruction or decomposition of the core material as a result of the high temperatures that prevail during that joining process. As a result of the decomposition or combustion of the core material during welding, gases or vapors can be released in an uncontrollable manner, making it impossible to obtain uniform, pore-free welds. Consequently, it is impossible to achieve a durable, mechanically stable joint. Furthermore, the decomposition of the core material in the vicinity of the weld seam effects a reduction in the mechanical properties of the core and so of the whole joint; sometimes it even leads to at least partial elamination of the outer layers from the core.